I'd love to hear about some folks experience with different yeasts. I've traditionally used the classic Fleischman's active yeast, and have recently used a starter I have, which yielded a nice sour flavor. But have not yet tried fresh yeast.

To the above, I would add that using natural leavening systems will yield substantial byproducts of fermentation that contribute to final crust flavor. Those flavors will be more complex and different than the flavors imparted by commercial yeast products. I think most people would say that they prefer the flavors imparted by natural leavening systems over the flavors of commercial yeast.